The discovery of high critical temperature ceramic compositions having superconducting properties is of recent origin. Originally, superconductivity was observed in mercury at 4 K by the Dutch scientist, Heike Onnes. The term, superconductivity, refers to the property wherein a normally resistive conductor abruptly loses all resistance to electrical flow at a specific temperature, called the critical temperature, T.sub.c. At this point, the resistivity of the normal conductor becomes zero, or superconducting. In more recent times, niobium metal alloys have been used in superconducting coils at temperatures up to 23 K. It has been believed that superconductivity above 23 K. was not possible. This belief was based on the theoretical work of Bardeen, Cooper and Schieffer (BCS theory-1946) which predicted such a limit. Several theoretical proposals were presented in the 1970's, suggesting that the critical temperature for superconductivity could be increased. However, the lack of any discoveries of superconductivity above 23 K solidified the belief that indeed this temperature could not be exceeded. Thus, in November, 1986, when Bednorz and Miller announced the discovery of a new ceramic superconducting compound based on lanthanum, barium, and copper oxides, whose critical temperature for superconductivity was close to 35 K., (G. Bednorz and A. Muller, Z. Phys., B64 189 (1986)), the declaration was greeted with considerable scepticism. Nevertheless, by the following month, the critical temperature, T.sub.c, for the onset of superconductivity was raised to nearly 80 K by C. W. Chu and coworkers (M. K. Wu, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett. 58 908 (1987)). This was achieved by changing the composition to yttrium barium copper oxide, approximated by the formula: EQU Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.6.3
This formula, determined experimentally, is not exactly stoichiometric. It is believed that this lack of specific nonstoichiometry contributes most to the onset of superconductivity. The so-called 1:2:3 compound, composed of Y-Ba-Cu-O atoms, is prepared by the solid state reaction of the requisite oxides, vis: EQU Y.sub.2 O.sub.3 +2BaO+3 CuO=2 YBa.sub.2 Cu.sub.3 O.sub.6.5.
It is now established (C. N. Rao et al, Nature, 327 185 (1987)) that high T.sub.c superconductivity in the Y-Ba-Cu-O system originates from a compound of stoichiometry: YBa.sub.2 Cu.sub.3 O.sub.7-.pi., where ".pi." is a value less than 1.0. This compound has the structure of the ideal perovskite, YBa.sub.2 Cu.sub.3 O.sub.9. Thus, the superconductor YBa.sub.2 Cu.sub.3 O.sub.7-.pi. has about 25% fewer oxygen atoms present in the lattice as compared to the idealized cubic perovskite structure. This massive oxygen deficiency means that instead of the conventional three-dimensional crystalline cubic-stacking array of the perovskite, a unique layered structure results. A loss of even more oxygen atoms in this structure gives rise to the semiconductor, YBa.sub.2 Cu.sub.3 O.sub.6. The chain of copper atoms associated with a chain of oxygen atoms is believed to be the key to superconducting behavior. Yet the above description is an idealized one and the actual distinct structural conformation has not yet been delineated. Note that there appear to be extra oxygen atoms in the superconducting unit cell, compared to that of the semiconductor.
To date, most of the high-T.sub.c superconducting ceramic compositions announced to date are based on cuprate compounds having Cu-O.sub.2 layers as part of the structure. Some of these have included:
Bismuth Strontium Calcium Copper Oxide EQU Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8+y EQU T.sub.c =114 1K.
Thallium Calcium (Barium) Copper Oxide
______________________________________ Tl Ba.sub.2 Ca Cu.sub.2 O.sub.7 Tl Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 Tl Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.11 Tl Ba.sub.2 Ca.sub.4 Cu.sub.5 O.sub.13 T.sub.c = 120 K. ______________________________________
Lead Strontium Lanthanide Copper Oxide EQU Pb.sub.2 Sr.sub.2 (Nd.sub.0.76 Sr.sub.0.24) Cu.sub.3 O.sub.8+x EQU T.sub.c =77 K.
In the last compound given, the CuO.sub.2 - sheets are present but there is also a PbO-Cu-OPb sandwich as well, not observed in ceramic superconductors heretofore. The copper ions in this sandwich are monovalent and each is coordinated, above and below, to two oxygen atoms in the PbO layers. The copper atoms in the CuO.sub.2 sheets have an average valence of about 2.25, which is consistent with previously discovered cuprate compounds, given above. However, the presence of Cu.sup.+ atoms lowers the average valence of copper ions in the new structure to below 2.0, which is atypical. Indeed, preparation conditions needed to prepare these compounds include a mildly reducing atmosphere so as not to oxidize Pb.sup.2+ to Pb.sup.4+.
There have also been some copper-free compositions announced, vis: EQU BaO- K.sub.2 O - Bi.sub.2 O.sub.3
This compound is said to become superconducting at about 30 K. While copper-oxide superconductors exhibit layered structures that carry current efficiently only along certain planes, this new material is a three-dimensional network of bismuth and oxygen with properties that are much less sensitive to crystallographic direction. It is hoped that compositions will be discovered in this system with temperature properties that rival those of copper-bearing compounds.
The main advantage to superconducting compositions with higher T.sub.c values is that they should perform better, i.e.--carry higher currents, when cooled to liquid nitrogen temperatures (78 K.). Superconducting ceramic compositions are normally prepared by weighing out specific quantities of selected oxides. The combination is thoroughly mixed by conventional means and then fired at elevated temperatures above about 950 .degree. C. The induced solid state reaction causes the formation of the desired ceramic composition and structure. Further annealing in an oxygen atmosphere has been shown to improve the superconducting properties of the Y-Ba-Cu-O compound. The powder so-produced is then processed by conventional means to form a bar (by compaction) which is then used as the superconducting medium.
There are many applications which require the generation of rapidly increasing magnetic fields. It is rather straightforward to produce a decaying magnetic field. This can be easily achieved by charging the electromagnet winding slowly, using a combination of low current and low voltage from an external power supply. By using an external or an internal resistance in series with the magnet windings, one can force the current to decay, thus producing the required decreasing magnetic field. A rapidly increasing magnetic field can only be conventionally generated by charging the magnet quickly but requires the use of a very large power supply. For example, a 10 megajoule magnet requires a 100 megawatt power supply to charge it in one millisecond. I have found that the same field increase can be achieved by the use of my new invention wherein high critical temperature superconducting coils are arranged so as to permit energy and power transfer between coils, thus permitting the generation of a rapidly increasing magnetic field. I have further discovered that this operation is within the scope of the instant invention and can be achieved without the use of an abnormally large megawatt power supply, as has been employed in the prior art.